An already provided diving boat is generally designed to keep the interior of the boat body substantially under the atmospheric pressure.
In the case of diving, the water pressure outside the boat body will increase by about 1 kg/cm.sup.2 per diving depth of 10m. That is to say, with the increase of the diving depth, the pressure difference between inside and outside the boat body will become larger and therefore the conventional diving boat has been required to have in the boat body a strength high enough to endure such pressure difference.
As a result, there have been problems that, when the structure is strengthened in order to obtain the required strength, the weight will increase, difficulties will be increased to transport and handle the boat, the cost will be high and it will be difficult for a general individual to purchase such diving boat.